Docs and examples

[smartalecH]

Here a some important areas that could benefit from improved documentation and examples:

• [ ] Vector field formulations -- discuss the nuances of each method, how the loss function is computed, etc.
• [ ] Convergence -- wrt vector field formulation, number of terms, etc.
• [ ] Sharp bits example -- (e.g. what happens when the wavelength is a multiple of the unit cell...) all the gotchas
• [ ] Explanation of types, calculation basics
• [ ] Scattering matrix fundamentals
• [ ] Field processing routines
• [ ] Errata